TORONTO — A series of blasts in Toronto’s financial district that shut down the area Monday and sent commuters scrambling was caused by an overheated hydro vault fire, said officials.

The first explosion was heard shortly after 5 p.m. and heavy black smoke was seen billowing from a set of grates outside a Royal Bank office building near King Street and Yonge Street.

The chaotic scene continued with sounds of underground blasts as officers herded crowds away from the scene.

“Did you hear those sounds? Those are explosions,” an officer was heard yelling to bystanders.

No one was injured, Toronto Fire Services said.

The incident shut down a nearby subway station and caused several streetcars to take detours. Police cruisers blocked off streets in the area and officers were seen wearing surgical masks as smoke hung in the air.

The Royal Bank office building and parts of an underground shopping concourse were evacuated due to heavy smoke, fire officials said.

Toronto Fire Platoon Chief Kevin Shaw said firefighters were able to contain the blaze within an hour. He added that once hydro workers were able to cut the power to the vault, firefighters would be able to go in and douse any remaining hot spots.

“It’s energized electrical equipment that’s in the vault, it overheats, starts melting down … so that’s where you’re hearing the crackle and the popping,” Shaw told reporters Monday night. “There (were) visible flames out of there probably a half hour ago, but we feel that it’s definitely under control now.”

Shaw said the fire’s cause would likely be determined once hydro crews could get access to the vault.

“It could be dampness, water or an aging hydro vault, or all of the above,” Shaw said. “It all leads up to one of these fires.”

He said he has seen worse hydro vault fires downtown, but added that crews had to take precautions because it happened during rush hour.

Toronto Hydro workers were still unable to get to the vault by 9 p.m. Monday, spokeswoman Tori Gass told reporters.

She said she could not provide any information about what caused the explosions because it was still to dangerous for hydro crews to go in to inspect.

“It’s going to be quite messy down there, quite toxic,” Gass said. “I would say we’re not going to have answers quickly, unfortunately.”

She said Toronto Hydro routinely checks vaults around the city to make sure they are safe, but she could not say when this vault was last inspected.